Magnetic sound reproducer



Nov. 8, 1932. P. L. JENSEN MAGNETIC souuv REPRODUQER Filed July 25,1931 2 Sheets-Sheet l jJ/zZ/e/zfw": Feteri. Jens n Nov. 8, 1932 P. L. JENSEN 1,886,316

MAGNETIC SOUND REPRODUCER Filed July 25. 1931 I 7 2 Sheets-Sheet 2 -27206/250/" Pete/"Z (Tense/z Patented Nov. 8, 1932 UNITED STATES PATENT OFFICE PETER L. JENSEN, OF LA GRANGE, ILLINOIS ASSIGNOR T0 JENSEN RADIO MANU- FACTURING 00., OF CHICAGO, ILLINOIS, A CORPORATION OF ILLINOIS MAGNETIC SOUND REPRODUGER Application filed July 25, 1931.

This invention relates to magnetic sound reproducers, and particularly to magnetic sound reproducers of the electro-dynamic 0r nun-ing coil type.

An object of my invention is to provide an electro-dynamic reproducer employing a permanent magnet for producing the necessary magnetic lines of force in the air-gap.

Another object of my invention is to provide an improved inner pole-piece construction, and particularly an inner pole-piece of sectionalized construction which will lend itself to assembly where the air-gap in the outer pole-piece or permanent magnet part is relatively small and where the distance between the two arms of the permanent magnet is less than the length of the inner polepiece.

Another object of my invention is to provide a magnetic pole construction which tends to concentrate as dense a magnetic field as possible in the air-gap and which eliminates in a great measure objectionable leakage of lines of force.

Another object is to provide means for holding the voice coil concentric and freely movable in the air gap.

Another object is the provision of an improved anchoring arrangement for the conehousing.

Another object is to exclude foreign-particles and prevent the same from lodging in the relatively close air-gap of a speaker of this sort, and particularly from lodging in this gap and interfering with the free movement of the voice coil.

Another object is the provision of an improved arrangement of and mounting for the speaker transformer of the device.

Other objects and advantages, as well as other improvements in structure and mode of operation, will beapparcnt from the following detailed description taken in connection with the accompanying drawings in which I have disclosed certain embodiments of the invention for the purpose of acquainting those skilled in the art with the manner of constructing and operating devices embodying the invention. I

Serial No. 553,091.

In the drawings: I Figure 1 is a section taken longitudinally along the axis of a device embodying the invent'on with the permanent magnet partially in elevation;

Figure 2 is a fragmentary section taken similarly longitudinally along the axis of another device showing the preferred shape of the air-gap and another manner of fastening the larger and generally conical ended inner pole-piece to the arm of the permanent magnet;

Figure 3 is a perspectiveview of the upper part of the inner pole-piece;

Figure 4; is a perspective view of the lower part of the inner pole-piece;

Figure 5 is a perspective view of the tube for excluding foreign particles from the airgap from below;

Figure 6 is a more or less diagrammatic view partially in section and partially in elevation, showing one manner of centering the upper part of the inner pole-piece con centrically with respect to the annular airgap between it and the outer pole-piece; and- Figure 7 is a similar view showing how the opening through which the lower part of the inner pole-piece is fastened to the lower arm of the permanent magnet may be similarly centered with respect to the air-gap forming opening in the upper arm of the magnet.

Broadly considered, the present invention comprises a permanent magnet 5 bent to the shape to give a minimum leakage of lines of force, which I find is substantially that shown in Figure 1.

The annular air-gap 6 is formed by boring the correct dimensional opening 7 in the upper arm 8 of the magnet 5, and the correct dimensional air-gap is obtained by providing the lower arm 9 of the magnet 5 with a special pole-piece arrangement designated generally at 10, the upper end of which extends up into the opening 7 in the upper arm 8, forming the annular air-gap 6 so that it will be equi-dimensional at all points.

Due to the extreme small clearance permissible between the voice coil 12 and the walls of the air-gap 6, it is necessary to space the inner pole-piece with great precision with respect to the outer or circumferential polepiece. It is also highly desirable, in order to reduce the reluctance of the magnetic circuit, to make the inner pole-piece 10 of large cross section where it is attached to the arm 9 of the permanent magnet 5 and to decrease gradually or by steps the area or cross section of the inner pole-piece as it extends toward and into the opening 7 in the arm 8 which forms the pole-piece of opposite polarity. As a permanent magnet is usually heavy and unyielding, it is impossible to insert a conical center or inner pole piece into the small airgap 6, and in order to permit this insertion and at the same time the large cross section of the inner pole-piece where it is attached to the arm 9, I make this inner pole-piece of sectionalized construction comprising an up per part 1 1 and a lower part 15. The lower part 15 has a flat and relatively large base 16 formed normal to its axis and seating upon the upper surface of the arm 9 of the permanent magnet 5. The outer periphery of this part 15 is cylindrical at 17 toward the relatively large base end, and upwardly of the cylindrical portion 17 the periphery of the inner pole-piece part 15 is of conical formation at 18 tapering upwardly toward the upper part 14. The relatively large cross section of the base portion of the part 15 where it is attached to the arm 9 reduces the reluctance of the magnetic circuit and the relatively large base surface 16 provides a more satisfactory seat when attached to the permanent magnet.

It will be noted that the opening 6 in the upper arm 8 of the magnet 5 is beveled or countersunk at 20 from the side away from the inner pole-piece 10. This is advantageous as it produces a concentration of lines of force into the air-gap and at the same time it keeps the leakage between the inner polepiece and the upper arm of the magnet at a minimum. In Figure 1 I have shown the countersinking of the upper end of the open ing 7 in the form of a flat bevel or taper.

In Figure 2 I have shown in section a preferred shape of air-gap. In this case a soft iron insert 21 is secured in an opening 22 in the upper arm 8' of the permanent magnet, and this insert 21 has the opening 7 which, with the upper end of the inner polepiece part 14' projecting into the opening, forms the annular air-gap 6. The insert 21 is dished downwardly at 23 to the opening 7 which disposes this opening at least partially below the-plane of the arm 8', and the upper surface of the insert 21 is provided with a downwardly dished concavity 24 for the purpose of minimizing leakage between the inner pole piece and the iron insertand concentrating the lines of force into the air gap 6. This form of outer pole-piece in conjunction with the arrangement of the inner pole-piece with respect thereto affords the least possible leakage between the inner and outer polepieces except in the air-gap proper 6' where the greatest possible magnetic density is desired. In addition, the soft iron insert 21 possesses less reluctance than the hardened magnet itself, and a desired gain in lines of force in the air-gap is noted.

In the usual types of electro-dynamic speakers employing an clectro-magnet for producing the constant magnetic field a relatively large top plate or top pole-piece is generally employed. This provides a relatively large seat for the cone housing and makes the entire construction quite rigid when the speaker is attached to the bafile by means of screws passing through the cone housing. In the present device this seat is relatively small compared with the diameter of the cone and the weight of the complete speaker. For this reason I have found it desirable to anchor the cone housing 25 to the permanent magnet 5 at a point located at as great a distance as practical from the apex of the cone. This is attained by bending the permanent magnet 5 into the shape shown in Figure 1 and fastening the cone housing 25 to the outer or upper arm 8 of the magnet 5 by means of a screw 26 passing through the outer periphery of the cone housing and through the arm 8 of the permanent magnet at a point close to the highest part of the magnet and near the periphery of the cone housing and engaging a nut 27 with the opposite end of the screw 26. The inner radial portion 28 of the cone housing has the opening 29 and may be fastened to the arm 8 of the magnet by screws 30 passing through openings in the radial wall 28 and through registering openings in the arm 8 and having threaded engagement in nuts 31.

It has always been a difiicult problem to prevent foreign particles from entering and lodging in the relatively close air-gap in dynamic speakers. These particles will interfere with the free movement of the voice coil 12 and this particular problem is even more difiicult where a permanent magnet is employed as the permanent magnet keeps the air-gap in a constant state of energizatlon and any chips or particles subject to magnetic attraction upon getting into close proximity to the instrument tend to migrate toward the airgap and become lodged permanently there. In order to keep the air-gap permanently clean I provide for screening all entrances to this part of the speaker in such manner and by using such materials that this screening does not affect the acoustic properties of the speaker in any way. For this purpose I use a tube 34 the length of which is the same as the distance between the two arms 8 and 9 of the permanent magnet. This tube 34 may be made of cardboard or any other non-magnetic material and its in side diameter is such that it fits snugly or tightly over the relatively large cylindrical portion 17 of the inner pole-piece, which cylindrical portion 17 forms the largest diameter of the inner pole-piece. 'In this'manner the tube 34 is held firmly in place and fitting snugly between the arms 8 and 9 it prevents the entry of foreign particles into the airgap from below or from the inner side. I

In Figure 1 I have also shown the usual openings 36 in the cone housing; these openings 36 being provided to relieve the air pressure on the back of the cone during operatlon of the instrument. I close these openings 36 by covering the same with cloth as indicated at 37, which cloth is of a character to permit a comparatively free flow of air or sound to pass through it but positively stops any particles of any material size from entering. Finally, I close the front of the cone to the air-gap 6 by fastening a light piece of similar cloth as indicated at 38 over the flexible spi der 39 which may be of thin sheet metal having openings 40. The flexible spider 39 is secured marginally to the cone or flexible dia phragm 42 at 43 and centrally the spider 39 is fastened by a screw 44 to the outer or upper part 14 of the inner pole-piece, there be ing a spacing sleeve 45 between the outer end of the inner pole-piece and the central part of the spider, 39 and the screw 44 passes through an opening in the center of the spider and through the spacing sleeve 45 and is threaded into the outer end of the outer part of the inner pole-piece centrally thereof. Instead of the arrangement above described I also contemplate closing this opening by forming the cone in such manner that a fiat apex is formed integrally with the cone and extending across the apex of the cone.

The generally conical diaphragm or cone 42 is attached around its outer periphery at 48 to the outer periphery of the cone housing 25 which is flanged at'49, and rings 50, with an intervening covering of -cloth' 51 which will screen out foreign particles without aifecting the acoustic properties of the device, are applied as shown. The inner truncated end of the cone or conical diaphragm is rigidly secured at 52 to an insulatlng sleeve 53 which carries the movable coil 12 constituting the voice coil of the instrument. This coil 12 is wound upon and preferably firmly cemented to the outer surface of the sleeve 53. The outer periphery of the cone is provided inwardly of the rings 50 with annular folds 54 for permitting the desired yieldability.

In the manufacture of permanent magnets it is difficult to hold the final shape of the magnet to very close tolerances. The bending of a comparatively heavy bar of steel. the chemical contents of which include 6% of chromium. and the successive heat treatments necessary for obtaining the proper hardening, all tend to make it difiicult to retain the screw 61 passes through the exact shape in two otherwise intendedly identical magnets. With this condition inmind I proceed-to locate the inner p0lepiece in exact relation to the circumferential polepiece in the following manner:

The permanent magnet 5 is provided with the relatively large air-gap forming opening 7 in its upperarm and this opening is cylindrical and of a diameter a few thousandths of an inch larger than the outside diameter of the voice coil 12. The lower arm 9 of the magnet is provided with an openin 60 for receiving a relatively heavy'screwolt 61, the shank of which passes freely through the opening 60 and is threaded into a correspondingly tapped opening at the axis of the inner pole-piece part 15 and securely fastens this part 15 to the arm 9, the pole-piece part 15 being shaped substantially as shown before being fastened in place. After this operation is completed, a drill, end reamer or the like 62 (Figure 6) provided with a gauge 63 closely fitting the opening 7 in the upper arm 8 of the magnet is passed down through the opening 7 with the gauge in place in the opening and maintaining the concentric disposition of the drill therein and the drill is then operated to drill or form a seat 64 in the upper end of the inner part 15. The size of the drill is of the exact size of the diameter of the pole-piece part 14 and slightly smaller than the inside diameter of the voice coil 12, whereby the seat 64 has a correct spaced relationship with respect to the opening 7 in the top arm 8 of the magnet. The pole-piece part 14 is then dropped into this seat and is thereby positioned exactly concentrically with respect to the opening 7, so that the annular air-gap 6 is equi-dimensional at all points and perfect alignment exists between the inner and outer pole-pieces and predetermined clearance is provided between the voice coil and the inner and outer pole-pieces. The lower end of the inner pole-piece part 14 may be secured in the recessed seat 64 in the upper end of the pole-piece part 15 by a setserew 65 or a plurality of such set screws.

The clearance between the shank of the screw-bolt 61 and the opening 60 in the arm 9 of the magnet permits the inner pole-piece bolt 61 to secure the assembly in adjusted position.

In the form of the invention illustrated in Figure 2 the bottom part 15' of the inner pole-piece is secured to the arm 9' from above instead of from below. as in Figure 1. In this case the opening 60 is tapped and the pole-piece part 15 from above with its head 66 countersunk inwardly of or below the bottom of the recessed seat 14 and the lower end of the shank For the purpose of centering the opening 60 of the embodiment of Figure 1, or the opening 60 of the embodiment of Figure 2,

with respectto the air-gap forming opening 7 in the outer or upper arm 8 of the permanent magnet, the opening 60 (Figure 7) may be formed by a-drill 70 provided witha gauge 72 closely fitting the opening 7 in the upper arm 8. The drill is' passeddown through the opening 7 with the gauge in place in the opening and maintaining the concentric disposition of the drill therein, and the drill is then operated to drill or form the opening 60 which may be threaded or not threaded, as desired.

It is understood that the present invention is not limited to speakers employing a cone as a sound emitter but may be used with other sound emitting systems. Due to the fact that dynamic speakers usually operate most efficiently when a low resistance voice coil is used, a speaker of this type is usually coupled to an electric out-put circuit by means of a speaker transformer. I have found it advantageous to mount the speaker transformer indicated at 7 5 in Figure 1 on the permanent magnet 5 as it usually forms an integral part of the speaker. The placing of the speaker transformer in the correct place on the permanent magnet 5 is important as the transformer contains a core of iron and if improperly placed will cause an objectionable leakage and lowering of the effective flux of the permanent magnet. I

have, therefore, mounted the speaker transformer 7 5 at a point on the permanent magnet where the magnetic potential is substantially zero. The point of so-called zero magnetic potential is the point of magnetic indifference, or that point between the two extremes where the attractive force, after continually diminishing as one proceeds from either pole, ceases altogether. One manner of determining this point is by scattering iron filings upon a sheet of paper and agitating it immediately above the magnet. It is to. be understood that where I refer in this specification and in the claims to locating the transformer at the point of substantially zero potential of the magnet, I do not intend to limit the location to the exact point of zero potential, but intend to include location at substantially that point or near it, as illus- The device may be provided with a cup- I shaped metal enclosure 80, slotted at 81 for the passage of the arms of the magnet into the same and secured at 82 to tabs struck out from the radial portion 28 of the cone housmg. I

It will be apparent to those skilled in the art that the teachings of the present invention are broader than the specific embodiments illustrated, and I do not intend, therefore, to be limited to the specific details shown and described.

I claim:

1. A permanent magnet electro-dynamic reproducer including a moving coil, a permanent magnet having a pole surrounding said coil, a cone attached to said coil, a housing for said cone, said magnet being formed to position a portion of the magnet in proximity to the periphery of the cone housing and means anchoring the periphery. of the cone housing to said proximate portion of the permanent magnet.

2. A permanent magnet electro-dynamic reproducer including a permanent magnet and 'a cone housing anchored to the permanent magnet in proximity to the periphery of the housing.

3. A permanent magnet electro-dynamic reproducer including a moving coil, a permanent magnet, a pole-piece cooperating with the said permanent magnet to form an air-gap for said coil, and a transformer connected to said coil and mounted upon said permanent magnet at the point of substan- .point of substantially zero potential.

5. In an electro-dynamic loud-speaker a permanent magnet for producing a constant magnetic field, a moving coil, a sound emltting system and a pole-piece consisting of 1 two parts, one with a relatively large cross section and the other with a smaller cross section, the smaller cross section pole-piece being fixedly seated in the pole-piece of larger cross section.

6. In an electro-dynamic reproducer, a pole-piece comprising two parts, one of said pole-piece parts being of relatively large cross section and'having a recess therein, and the other pole-piece part being of smaller cross section and fixedly seated in said recess.

7. In a device of the class described, a permanent magnet having a pair of spaced poles, one provided with an opening, an inner pole-piece part of cross section larger than said opening secured to the other pole scopic engagement with the inner pole-piece.

of said magnet, and a second inner pole-piece part of smaller cross section fixedly seated in said first inner pole-piece part and extending into the opening in the adjacent pole of the permanent magnet.

8. A pole-piece comprising a pair of pole.- piece parts, one having a recess and tapered toward a base of relatively large cross section, and the other of smaller cross section and having one end secured in the recess in said first pole-piece part.

9. In a device of-the class described, a permanent magnet having a pair of spaced poles, one provided with an opening for co-operation with an inner pole-piece to form an air gap, and an inner pole-piece of a length greater than the distance between said arms and having a portion of cross section greater than the diameter of said opening and a portion of cross section less than the diameter ofsaid opening, said inner pole-piece being sectionalized to permit attachment of the larger cross section portion of said inner pole-piece to one of the arms of the permanent magnet and positioning of the smaller cross section portion of said inner pole-piece in the other arm of the permanent magnet.

10. In a device of the class described, a permanent magnet having a pair of spaced poles, one provided with an opening for cooperation with an inner pole-piece to form an air-gap, and an inner pole-piece of a length greater than the distance between said poles and having a portion of cross section greater than the diameter of said opening and a portion of cross section less than the diameter of said opening, said inner pole-piece being sectionalized to permit attachment of the larger cross section portion of said inner pole-piece to one of the arms of the permanent magnet and positioning of the smaller 8 cross section portion of said inner pole-piece in the opening in the other arm of the magnet, a moving coil in the air-gap, and a transformer connected to said moving coil and mounted upon the permanent magnet at the position of substantially zero potential.

.11. In a device of the class described, an outer pole-piece having an opening therein, an inner pole-piece extending into said opening with an annular space forming an airgap between it and the outer pole-piece and a shield positioned over the inner pole-piece to prevent the entrance of foreign particles to said air-gap.

12. In aflevice of the class described, an

outer pole-piece having an opening therein,

an inner pole-piece extending into said opening with an annular space forming an air-gap between it and the outer pole-piece and a shield positioned over theinner pole-piece to prevent the entrance of foreign-particles to said air-gap, said shield comprising a nonmagnetic tube having relatively snug tele- 13. In a device of the class described, a permanent magnet having a pair of spaced poles, one having an opening therein, an inner pole-piece extending into said opening with an annular space forming an air-gap between it and the adjacent pole of the permanent magnet and a shield positionedover the inner pole-piece to prevent the entranceof foreign particles to said air-gap, said shield comprising a non-magnetic tube having relatively snug telescopic engagement with the inner pole-piece and of a length substantially equal to the distance between the poles of the permanent magnet.

14. In a device of the class described, a permanent magnet having a pair of arms, one provided with an opening, and a pole-piece attached to the other arm of the magnet and extending into one end of said opening with a space forming an air-gap between it and the surrounding portion of the adjacent arm of the magnet, the opposite end of said opening being enlarged to reduce leakage of magnetic flux and to concentrate a relatively dense magnetic flux in the air-gap.

- In witness whereof, I hereunto subscribe my name this 21st day of July, 1931.

PETER L. JENSEN. 

